The present invention relates to a process for preparing polymers by means of anionic polymerization. More particularly the present invention relates to a new method for the termination of such anionic polymerization reactions or for treating coupled reaction products of such polymerization reactions.
It is previously known in the art to prepare polymers of anionically polymerizable monomers, especially monovinylidene aromatic monomers and alkadiene monomers via anionic polymerization. Highly useful polymers are prepared in the form of block copolymers containing one or more blocks of a monovinylidene aromatic polymer and one or more blocks of an alkadiene polymer. Suitably such polymers are prepared in an organic solvent and are usefully employed in adhesive formulations, as modifiers for thermoplastic resins and asphalt or bituminous compositions or in solvent containing cements or mastic formulations. The polymers containing residual unsaturation may be hydrogenated if desired to produce products having improved weathering resistance.
The initiators used in such anionic polymerizations are typically very strong bases. Examples are the alkali metal organyl compounds, particularly alkali metal alkyls, especially sodium and lithium alkyls. After polymerization is complete, the terminal monomer group of the living polymer anion must be terminated. Termination may occur through a coupling reaction by means of a coupling agent or in the event a coupled compound is not desired by the use of suitable proton donating agents such as an organic alcohol, ammonia, amines or even water to terminate the living anion. However, the remnant formed by this termination is itself a basic species. In the industrial preparation of polymers via anionic polymerization, especially the preparation of block copolymers of monovinylidene aromatic monomers and alkadienes it is often desirable to include an antioxidant in the polymeric syrup to prevent oxidative and mechanical degradation of the polymer during devolatilization and finishing. However, many of the suitable antioxidants employed in such formulations are sensitive under basic conditions. That is, such antioxidants react with basic species thereby forming undesirable products. Alternatively under basic conditions the antioxidant may be inhibited in its ability to prevent oxidative degradation of the resulting polymer.
Consequently products which have been neutralized by the use of organic alcohols, especially such products further containing an antioxidant, have been found to be lacking in both color retention and in aging stability. Such polymeric products have been found to change in melt viscosity after further mechanical and thermal treatment and to be marked by an absence of clarity as a result of increased haze. In addition such polymeric products tend to have increased yellowing. Also, physical properties, such as ultimate tensile strength, are adversely affected.
In U.S. Pat No. 4,415,695 it is proposed to employ boric acid as a terminating agent in an anionic polymerization. Disadvantageously when boric acid is employed as a terminating agent in the preparation of a block copolymer of a monovinylidene aromatic monomer and an alkadiene, the resulting product still possesses an undesirable change in melt viscosity upon thermal aging.
It would be desirable if there were provided an improved process for preparing polymers by means of anionic polymerization techniques employing a terminating agent which does not adversely affect the polymer properties.
It would also be desirable if there were provided an improved technique for treating terminated reaction products prepared by anionic polymerization that allows for the preparation of polymers having improved clarity and other polymer properties.
It would be desirable if there were provided an improved technique for treating coupled reaction products prepared by anionic polymerization that allows for the preparation of polymers having improved clarity and other polymer properties.
Finally, it would be desirable if there were provided adhesives containing as one component the improved block copolymers of monovinylidene aromatic monomers and alkadienes prepared according to the present invention.